This invention relates to a process for the manufacture of silyl ketene acetals. More specifically, this invention relates to a means of producing high-purity silyl ketene acetals from the reaction of dialkyl dialkylmalonates or bis(trialkylsilyl) dialkylmalonates and triorganohalosilanes in the presence of an alkali metal.
The first reference to preparation of silyl ketene acetals (SKA) was in the late-1950's by Petrov et al., J. Gen. Chem. (USSR), 29(1959), pp. 2896-2899. This reference and most of the other references to the art deal with chemical species of the general formula, ##STR1## These organosilane intermediates are of value because of the ability to further react the SKA to prepare organic compounds which would be difficult to synthesize by other means. A very recent application is the use of the SKA as acrylate polymerization initiators. This concept known as Group Transfer Polymerization (GTP) was developed by DuPont and is disclosed in three U.S. patents--U.S. Pat. No. 4,414,372, Farnham et al., issued Nov. 8, 1983; U.S. Pat. No. 4,417,034, Webster, issued Nov. 22, 1983; and U.S. Pat. No. 4,508,880, Webster, issued Apr. 2, 1985.
Four procedures for preparing silyl ketene acetals are known in the art. The first general route to SKA is the reaction of an ester of a carboxylic acid with an appropriate metal reagent to form a metal enolate ion and subsequent reaction of the enolate ion with an organochlorosilane. Ainsworth et al., J. Organometallic Chem., 46(1972), pp. 59-71, describe the preparation of an SKA via the reaction of esters of carboxylic acids with lithium diisopropylamide, followed by reaction with trimethylchlorosilane. Kita et al., Tetrahedron Letters, 24:12 (1983), pp. 1273-1276, discloses a similar procedure to prepare bifunctional SKA. Brown, J. Org. Chem., 39:9(1974), pp. 1324-1325, describes the preparation of metal enolate ions by reacting potassium hydride in tetrahydrofuran with a carbonyl compound, followed by reaction with excess triethylamine and trimethylchlorosilane.
Kuo et al., Chemical Communications, (1971), pp. 136-137, discloses the preparation of silyl ketene acetals of the formula, EQU R.sup.1 R.sup.2 C.dbd.C[OSi(CH.sub.3).sub.3 ].sub.2,
wherein R.sup.1 and R.sup.2 are hydrogen, methyl, t-butyl, and phenyl. The silyl ketene acetal is prepared by the reaction of the corresponding carboxylic acid or silyl ester of a carboxylic acid in contact with lithium diisopropylamide, trimethylchlorosilane, and tetrahydrofuran. Yields of the desired silyl ketene acetal of from 29 to 85 percent are disclosed. Kuo et al., are silent as to whether or not the yield figures disclosed are calculated by analysis or physical isolation and separation.
In a second general procedure, silyl ketene acetals are prepared by the hydrosilation of esters of carboxylic acid with organohydrosilanes. Petrov et al., J. Gen. Chem. (USSR), 29(1959), pp. 2896-2899, described the platinum-catalyzed reaction of methyl methacrylate with triethylsilane. Ojima et al., J. Organometallic Chem., 111(1976), pp. 43-60, studied the use of tris(triophenylphosphine)rhodium chloride as a catalyst. Howe et al., J. Organometallic Chem., 208(1981), pp. 401-406, and Yoshii et al., Chem. Pharm. Bull., 22(1974), pp. 2767-2769, describe yields of 70-75% SKA from the reaction of (C.sub.2 H.sub.5).sub.3 SiH and methyl methacrylate using organophosphorous complexes of rhodium as a catalyst. Quirk et al., in European Patent Application 0184692, published June 18, 1986, discloses o-silylated ketene acetals and enol ethers and a process for their preparation from the reaction of acrylate esters and silanes or siloxanes in the presence of a rhodium catalyst.
In a third procedure Ishikawa et al., in U.S. Pat. No. 4,482,729, issued Nov. 13, 1984, describe the preparation of a fluoroalkyl silyl ketene acetal by the reaction of a fluorinated carboxylic acid ester with trimethylsilyl trifluoromethanesulfonate.
The fourth procedure involves the alkali metal reduction of disubstituted malonates in the presence of trimethylchlorosilane to produce a silyl ketene acetal. Kuo et al., Chemical Communications, (1971), pp. 136-137; and J. Am. Chem. Soc., 94: 11 (1972), pp. 4037-4038, disclose the preparation of silyl ketene acetals of the formula, EQU R.sup.1 R.sup.2 C.dbd.C(OR.sup.3)OSi(CH.sub.3).sub.3,
from the reaction of a dialkyl dialkylmalonate with trimethylchlorosilane in the presence of sodium metal, wherein the R.sup.1 and R.sup.2 are methyl, ethyl, or phenyl; and R.sup.3 is methyl or ethyl. The use of xylene and ammonia as solvents is disclosed by Kuo et al. Further, Kuo et al., discloses that the course of the reaction is different when xylene is used as a solvent as compared to the case when liquid ammonia is present. This result reported by Kuo et al., indicates that this reaction is solvent dependent. Nowhere does Kuo et al., disclose that silyl ketene acetals of the formula, EQU R.sup.1 R.sup.2 C.dbd.C[OSi(CH.sub.3).sub.3 ].sub.2,
can be prepared from malonates.